1. Field of the Invention
The present invention relates to a fixed-point cell required to calibrate a thermometer such as, for example, a radiation thermometer used in a high-temperature region exceeding 1100xc2x0 C., and to a thermometer calibration method and fixed-point temperature realizing apparatus using the cell.
2. Description of the Prior Art
When thermometers are calibrated, at or above a room temperature region, a freezing point or a melting point of a metal is employed as a defining fixed-point temperature of the International Temperature Scale of 1990 (ITS-90). A fixed-point cell is used as a method of realizing the fixed-point temperature. Carbon crucibles, in which pure metal is cast as a fixed-point material of high melting point, have been usually used as the fixed-point cell. A thermometer is calibrated utilizing the fact that when the liquid phase and the solid phase of the fixed-point material coexist in the crucible, the temperature of the cell is not varied due to the latent heat of the melting. This is realized by placing the fixed-point cell in a temperature-variable furnace and observing the temperature variation of the cell when an environmental temperature is increased and decreased (see Chapter 7 of xe2x80x9cNew Edition of Proper Use of Thermometer,xe2x80x9d edited by Japan Electric Instrument Manufacturers"" Association, published by Nihon Kogyo Publishing Co. (1997)).
The maximum fixed-point temperature is the copper point of 1085xc2x0 C.; at temperatures higher than the copper point the temperature scale is defined by extrapolation. In the temperature region above the copper point the temperature scale is maintained by utilizing a radiation thermometer calibrated at a fixed point below the copper point, or by transferring the radiance temperature to the current of a tungsten ribbon lamp.
Attempts to realize a fixed-point temperature in the temperature region above the copper point include attempts to realize the freezing point of palladium (freezing point: 1550xc2x0 C.) and platinum (freezing point: 1770xc2x0 C.), and an example where the fixed-point temperature was measured by melting them using an alumina crucible has been reported (see T. J. Quinn, T. R. D. Chandler: Temperature, Its Measurement and Control in Science and Industry, H. H. Plumb, (ed.), Vol. 4, Part 1, p. 295, Pittsburgh: Instrument Society of America (1972), P. B. Coates, T. R. D. Chandler, J. W. Andrews: High Temperature and High Pressure, Vol. 15, p. 573 (1983)).
There has also been reported an attempt at obtaining a fixed-point temperature by using tungsten as a crucible material, melting alumina therein, observing the melting and freezing thereof at 2050xc2x0 C. with a radiation thermometer and using that as a fixed-point temperature (see H. Sakate, F. Sakuma, A. Ono: Metrologia, Vol. 32, p. 129 (1995)).
On the other hand, an attempt has been made to use metal-metal eutectics as a fixed-point temperature material. It has been reported that a fixed-point temperature was realized by casting copper-silver eutectics or copper-aluminum eutectics in a carbon crucible and observing the melting and freezing thereof (Itoh, Papers Of The Society of Instrumentation and Control Engineers, Vol 19, No. 12, p. 978 (1983)).
One or the present inventors has proposed a fixed-point cell comprising a crucible composed of carbon, a fixed-point material enclosed in the crucible that is a eutectic structure of carbon and metal, a fixed-point temperature realizing apparatus using the fixed-point cell comprising a furnace having the fixed-point cell disposed therein for increasing and decreasing an environmental temperature of the cell and a thermometer to measure temperature variations in the cell, and a calibration method for calibrating the thermometer based on the measured temperature variations (JP-A 2000-180272).
The maximum temperature of conventional fixed-point cells which use carbon and in which pure metal is cast is the copper point of 1085xc2x0 C. (FIG. 2). This is because when a metal having a higher melting point is melted in a carbon crucible, the carbon is dissolved in the metal, reducing the purity of the metal and thereby lowering the freezing point.
Thus, there is no fixed-point cell above the copper point, so the temperature scale is maintained by a ribbon lamp or radiation thermometer. However, the dependency on extrapolation of the temperature scale thus maintained greatly reduces the accuracy of the temperature scale. For example, in the case of calibration traceability of radiation thermometers in Japan, the provided accuracy is 0.3xc2x0 C. at 1085xc2x0 C., decreasing sharply to 4xc2x0 C. at 1600xc2x0 C. and to 8xc2x0 C. at 2000xc2x0 C.
Moreover, setting a higher-accuracy scale, using a high-accuracy radiation thermometer, for example, together with the precise evaluation of the thermometer characteristics and the like, involves a massive effort. However, since it still depends on extrapolation, the resulting uncertainty in temperature scale realization and maintenance is still in the region of 0.8xc2x0 C. at 2000xc2x0 C.
Also, since the emissivity of the ribbon in a ribbon lamp is not 1, correction is required depending on the measurement wavelength. In addition to the difficulty of using a ribbon lamp with high accuracy, the ribbon has to be sealed in an inert gas atmosphere since the tungsten vaporizes at temperatures above 2000xc2x0 C. As a result, stable ribbon lamp characteristics cannot be obtained due to the convection of the inert gas.
For realizing the palladium point or the platinum point, an alumina crucible can be used in the measurement for a short period. However, it is not practical for repeated use because the alumina is brittle and susceptible to thermal shock, making the crucible thus used prone to breakage, while a further problem is that owing to metal contamination arising from the alumina reduction, the crucible has to be used in an oxidizing atmosphere.
The method of melting alumina in a tungsten crucible has low realizability. The reasons for this include the poor workability of tungsten, the difficulty of sealing molten alumina into a lateral crucible, and the fact that sufficient accuracy cannot be obtained since a blackbody cavity whose emissivity is near to 1 cannot be formed due to the low emissivity of the tungsten.
The method of using metal-metal eutectics serves the purpose of increasing the number of fixed-point temperatures in a temperature region lower than the copper point. When the same method is used in a temperature region higher than the copper point, a fixed-point temperature cannot be realized because a freezing point is unavoidably lowered by the dissolution of carbon.
In the case of the method using a fixed-point cell comprising a crucible composed of carbon and a fixed-point material in the crucible that is a eutectic structure of carbon and metal, it is only possible to realize a fixed-point temperature up to 2732xc2x0 C. (FIG. 3). As a practical problem, osmium oxide is a colorless, highly-toxic gas. Considering its unsuitability as a fixed-point substance, as a fixed-point temperature using a eutectic structure of carbon and metal, the maximum temperature is 2474xc2x0 C. using a eutectic structure of carbon and rhenium.
An object of the present invention is to overcome the above shortcomings of the prior art by providing a fixed-point cell that extends the fixed-point temperature region to the temperature region exceeding 2500xc2x0 C. and enables the achievement of high calibrating accuracy in the calibration of all thermometers that are used in a high temperature region such as radiation thermometers and the like, and a thermometer calibration method and fixed-point temperature realizing apparatus using the above cell
To attain the above object, the present invention provides a fixed-point cell comprising a crucible composed of carbon and a fixed-point material enclosed in the crucible, wherein the fixed-point material is a eutectic structure of carbide and carbon.
A fixed-point temperature realizing apparatus according to the present invention comprises a fixed-point cell comprising a crucible composed of carbon and a fixed-point material enclosed in the crucible that is a eutectic structure of carbide and carbon, and a furnace having the fixed-point cell disposed therein for increasing and decreasing an environmental temperature of the cell and a thermometer to measure temperature variation in the cell.
A calibration method for calibrating a thermometer according to the present invention, the method comprising enclosing a fixed-point material that is a eutectic structure of carbide and carbon in a crucible composed of carbon to form a fixed-point cell, raising and lowering an environmental temperature of the cell, measuring the temperature variation of the cell with a thermometer to be calibrated, and calibrating the thermometer based on the measured temperature variation.
As the carbide, there may be used a carbide of any of boron, molybdenum, vanadium, titanium, zirconium, hafnium, niobium, tantalum, tungsten, and rare earth elements.
The thermometer may be a thermometer used for measuring high temperatures such as a radiation thermometer.
As described in the foregoing, accurate thermometer calibration can be effected even in a temperature region exceeding 2500xc2x0 C. by using a fixed-point material having a eutectic structure of carbide and carbon.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.